Sputter deposition is a widely used technique for depositing thin films. In a typical sputter-deposition system, a power supply is used to apply a voltage to a target cathode, hereafter referred to as cathode voltage. The applied cathode voltage partially ionizes an inert gas in a vacuum chamber of the sputter-deposition system, creating a plasma. The plasma contains positively charged ions, which are attracted to the negatively charged target cathode and accelerate towards it. When the positive ions collide with the target cathode, target material is sputtered from the target cathode. The sputtered target material deposits as a layer on a substrate.
To expand the range of materials that can be deposited as a layer, a reactive gas, such as oxygen or nitrogen, can be introduced into the vacuum chamber of the sputter-deposition system. Such a reactive-sputtering technique produces a layer composed of a material, for example, an oxide or a nitride, that is derived from the target material by chemical reaction with the reactive gas.
For many applications, such as optical coatings and filters, thin films having one or multiple predetermined refractive indices are required. For example, it is advantageous for an antireflection coating to have a refractive index as close as possible to the value that minimizes reflection at an interface. For another example, a Rugate notch filter requires a plurality of refractive indices that vary continuously along the thickness of the filter. For such applications, the required refractive indices are often intermediate to those accessible by common sputtering techniques.
When a single target material is sputtered, the following approaches can be taken to deposit a layer having such an intermediate refractive index. In a first approach, the cathode power or the substrate temperature can be adjusted to vary the microstructure and, hence, the refractive index of the deposited layer, as described in U.S. Pat. No. 6,506,289 to Demaray, et al. and U.S. Pat. No. 6,666,958 to Yoshikawa, et al., for example. In a second approach, the composition of the single target material can be adjusted, for example, by doping, to vary the composition and, hence, the refractive index of the deposited layer, as described in U.S. Pat. No. 6,506,289 to Demaray, et al. In a third approach, when the single target material is sputtered in the presence of a reactive gas, the flow rate of the reactive gas can be adjusted to vary the composition and the refractive index of the deposited layer, as described in U.S. Pat. No. 6,506,289 to Demaray, et al. and U.S. Pat. No. 6,666,958 to Yoshikawa, et al. In a fourth approach, when a single target material is sputtered in the presence of a mixture of reactive gases, the ratio of the reactive gases can be adjusted to vary the composition and the refractive index of the deposited layer, as described in U.S. Pat. No. 6,217,719 to Kanazawa, et al., U.S. Pat. No. 6,506,289 to Demaray, et al., and World Patent Application No. WO 1998/37254 to Placido.
Alternatively, target materials having different compositions can be sputtered at the same time, i.e. cosputtered, to deposit a layer composed of a mixture of materials. The refractive index of the deposited layer, which is a composite of the refractive indices of the materials in the deposited layer, can be varied by adjusting the ratio of the materials in the deposited layer.
In a first cosputtering technique, a single target cathode including a plurality of regions, each of which comprises a target material having a different composition, is sputtered. Variations on this technique are described in U.S. Pat. No. 4,468,313 to Okumura, et al., U.S. Pat. No. 4,505,798 to Ramachandran, et al., and U.S. Pat. No. 6,692,618 to Dubs, for example. However, this cosputtering technique has the disadvantage that, in many instances, the fabrication of single target cathodes comprising a plurality of target materials is difficult or impossible.
In a second cosputtering technique, a plurality of target cathodes, each of which comprises a target material having a different composition, are simultaneously sputtered. Variations on this technique are described in U.S. Pat. No. 3,502,562 to Humphries, U.S. Pat. No. 4,252,626 to Wright, et al., and U.S. Pat. No. 6,800,183 to Takahashi, for example. This cosputtering technique allows layers composed of a mixture of materials to be deposited by using conventional target cathodes comprising single target materials.
When a plurality of target cathodes, each of which comprises a target material having a different composition, are cosputtered, the composition and the composite refractive index of the deposited layer can be adjusted by independently changing the cathode powers supplied to each of the target cathodes, as described in U.S. Pat. No. 5,225,057 to Lefebvre, et al., U.S. Patent Application Publication No. 2004/0182701 to Miyamura, and an article entitled “Cosputtered films of mixed TiO2/SiO2” by Laird and Belkind (Journal of Vacuum Science and Technology A, 1992, Vol. 10, pp. 1908-1912), for example. This approach has the disadvantage of requiring a sputter-deposition system in which each of the target cathodes is connected to a separate power supply.
When two target electrodes, each of which comprises a target material having a different composition, are sputtered using a single alternating-current (AC) power supply, the composite refractive index of the deposited layer can be adjusted by changing the AC voltage applied across the target electrodes, as described in U.S. Pat. No. 6,585,871 to Anzaki, et al. However, as only one target electrode is sputtered at a time in this approach, the rate at which layers can be deposited is relatively low.
An object of the present invention is to overcome the shortcomings of the prior art by providing a simple method and sputter-deposition system for depositing a layer composed of a mixture of materials and having a predetermined refractive index. The present invention has the advantage of operating a plurality of target cathodes, each of which comprises a target material having a different composition, with a single direct-current (DC) power supply. The use of a single DC power supply represents a great simplification over the conventional use of a plurality of power supplies.
The present invention also recognizes that, rather than by individually adjusting operating parameters of each target cathode, the composite refractive index of the deposited layer can be controlled by adjusting an operating parameter of the plurality of target cathodes. The operating parameter can be cathode power, cathode voltage, or cathode current. Alternatively, the operating parameter can be an angle between a cathode support and a substrate, or a flow rate of a reactive gas.